1. Field of the Invention
This invention relates to microstrip transmission lines operating in the millimeter wave region of the frequency spectrum and more particularly to a microstrip Y-junction circulator for use with such microstrip transmission lines.
2. Description of the Prior Art
Y-junction circulators are non-reciprocal coupling devices having three ports which provide signal transmission from one port to an adjacent port while decoupling the signal from the remaining port. They are used in radar system front ends as duplexers to couple the transmitter and receiver to the single radar antenna. They are also used in many other applications such as signal generator protection circuits and transmitter injection locking circuits, for example. With the great increase in use of planar circuitry using microstrip transmission lines in millimeter wave frequency application because of the resulting reduction in size and weight of the equipment involved, a need has arisen for a Y-junction circulator which is suitable for use with such planar circuitry and microstrip transmission lines.
Conventional millimeter wave microstrip circulator designs generally utilize a small ferrite disc or "puck" which has metallized ends and which is disposed in a hole in the microstrip transmission line substrate at the point where the microstrip lines to be coupled meet. The puck has a thickness which is equal to the thickness of the microstrip transmission line substrate so that the metallized ends of the puck may be electrically connected to the microstrip conductors and the metal ground plane of the transmission line. When a unidirectional magnetic field is applied between the ends of the puck, a clockwise or counterclockwise non-reciprocal coupling action is produced between the microstrip lines which are joined at the puck. The clockwise or counterclockwise coupling direction may be reversed by reversing the direction of the applied magnetic field. A circulator of this type is shown and described in U.S. Pat. No. 3,456,213 issued July 15, 1969.
The manufacturing and assembly costs of the puck-type circulators are relatively high because the ferrite puck must be fitted into the substrate hole with a very close tolerance fit to minimize line impedance variations and to reduce insertion losses. Additionally, if the dielectric constant of the microstrip substrate is different from the dielectric constant of the ferrite, a matching transformer configuration is required which further increases the aforementioned costs. Furthermore, the ferrite puck arrangement is not readily adapted to the monolithic design and automated assembly techniques which must be utilized in the fabrication of microstrip circuits in order to reduce their complexity and cost.